1. Field of the Invention
The present invention relates to a method for fabricating Fabry-Perot interferometer elements (etalons) and more particularly to a photolithographic method for fabricating very flat, polished and highly uniform, free-standing metallic meshes. Such metallic meshes may be utilized, for example, as elements in Fabry-Perot or Michelson interferometers, as filters to filter radiation, liquids or particles, as optical/infrared reflectors, as optical/infrared beam splitters or dichroics, as optical polarizers and in other like devices.
2. Description of the Prior Art
Metal films on dielectric coatings are able to provide high reflectivity surfaces, but in the infrared spectral region they have fundamental absorption losses of several percent which can limit their usefulness in associated systems. One such system is a Fabry-Perot interferometer, in which the radiation reflects tens or hundreds of times from the surfaces, suffering absorption losses at each pass. In the far-infrared portion of the spectrum, beyond about 30 microns, the limitations of metal films can be overcome by using metal meshes which resonantly reflect the radiation with losses of less than 1%. However, fabrication of such metal meshes, especially free-standing meshes, has been difficult.
For proper operational use in Fabry-Perot interferometers, such metal meshes should meet the following requirements. They must be made to specific, uniform and exacting tolerances, as specified by the theory which models wire widths, periodicity and thickness. The squareness of the holes and the steepness of the sides appear to be additional important parameters. At far-infrared wavelengths, the metal meshes must be mechanically strong so that they will not tear or stretch with mounting or handling, and this mounting must be capable of pulling the mesh very flat to achieve high "flatness finesse". The root-mean-square (rms) surface roughness of the reflecting surface of the mesh must be low (e.g. less than about 300 angstroms (.ANG.)) for high transmission. The mesh surfaces must be capable of having a gold or copper coating on all of their areas for high electrical conductivity, which parameter helps provide low absorption. The metal meshes must be made in a clean environment or be capable of a thorough cleaning so that the radiation sees few impurities. Finally, the metal meshes also must withstand cryogenic temperatures.
Unfortunately, the prior art metal meshes do not meet all of the above described requirements.
One presently used method for fabricating metal meshes involves the etching of bi-metallic layers. In this first type, two layers of metal are electroformed on a block which can be removed after the mesh is formed. The top metal in the bi-metal block is patterned by some lithographic technique. Then the block is removed and the bottom metal, which operates as a support metal for the top metal, is selectively etched in some regions to provide a free-standing grid or mesh of the top metal. The surface of the patterned top metal is usually very rough. Therefore, a mesh made with such a patterned top metal would be unsuitable for use as a Fabry-Perot interferometer element. On the other hand, the surface of the bottom metal that is in contact with the block can be made smooth. However, the bottom metal is too thick to be patterned with good dimensional control.
Another presently used method for fabricating metal meshes is by using a ruling engine for pattern formation. Such a fabricated metal mesh tends to be irregular in shape. More importantly, the finished pattern of this fabricated metal mesh is usually very rough and non-uniform, which degrades both the transmission and the resolution of the Fabry-Perot interferometer metal mesh element or etalon.